This invention relates to circuitry for driving the stator coil of a stepper motor and more particularly for providing two amplitudes of bipolar drive current to the coil depending on the instantaneous torque requirements of the motor.
Typically, stator coils of bipolar stepper motors are driven by bridge amplifiers which effectively commutate the polarity of a supply potential across the coil. These circuits require at least four driver transistors to perform the requisite switching. In addition in order to change the coil current amplitude, for example, when the motor is in a hold mode as opposed to a drive mode, some means must be provided to alter the drive potential, alter the driving impedance, or to chop the drive signal to provide selectable "average" drive parameters, etc. Circuitry to permit such coil current adjustment usually complicates the system, adds to system cost, adds to system bulk and adds to the energy dissipated by the driver circuitry.
Certain stepper motor applications require that the motor operate in the high torque mode for relatively short intervals with the major portion of its operation being that of stepping at a slow rate so that coil current demands are primarily holding currents. For example, in a video disc player a stepper motor may be used to drive a carriage which carries the recorded signal recovery transducer. At the beginning of play it is desired that the carriage move rapidly to seek the beginning of the recorded track on the record. The motor step rate may be at 300 steps per second, i.e., the motor must operate at high torque. However, during normal playback the carriage is required to move slowly in consonance with the transducer following the record track. In this mode the motor may step at 1-2 steps per second and will be operating in a low torque mode.
Generally both the motor and the motor driving circuitry may be overdriven for short intervals without damage to either. Thus, for example, a 6 volt motor may be energized with 20 volts for periods of time too short to cause heating. The present inventor took advantage of this feature to design a bipolar stepper motor drive circuit which provides high torque for particular step rates, a low value of holding current and is less complicated and less costly than conventional bridge drive circuitry with similar drive features.